1. Field of the Invention
The invention relates to the manufacture of thin steel sheet which can be drawn. More specifically, it relates to strip and sheet made of ultralow-carbon and low-carbon ordinary steel.
2. Description of the Background
Conventionally, thin strip (thickness from 0.5 to 1.5 mm) made of carbon steel intended to be drawn, and used for example in the motor-vehicle industry, is obtained by the following manufacturing line:
continuous casting of slab approximately 200 mm in thickness; PA1 hot rolling of said slab until strip about 4 mm in thickness is obtained; PA1 cold rolling, annealing (box or continuous annealing) and passage through a skin-pass mill of said strip (operations referred to by the term "cold treatments," even if for some of them, such as the annealing, a reheat is necessary) the strip then being cut up in order to obtain sheet. PA1 a steel strip from 1.5 to 10 mm in thickness having a composition in percentages by weight that comprises, consists essentially of, or consists of: carbon less than 0.1%, manganese from 0.03 to 2%, silicon from 0 to 0.5%, phosphorus from 0 to 0.1%, boron from 0 to 0.002%, titanium from 0 to 0.15%, iron and impurities resulting from the smelting, is cast directly from liquid metal; PA1 said strip, in the austenitic phase, then undergoes a first hot rolling operation in one or more steps at a temperature of between 950.degree. C. and the Ar.sub.3 temperature of said strip with an overall reduction ratio of at least 10%; PA1 said strip, in the ferritic phase, then undergoes a second hot rolling operation in one or more steps at a temperature below 850.degree. C., with an overall reduction ratio of at least 50% in the presence of a lubricant, so as to obtain a hot-rolled strip having a thickness of less than or equal to 2 mm; and PA1 said strip then undergoes a complete recrystallization over its entire thickness by a soak between 700 and 800.degree. C.
The composition of this sheet may be summarized as follows (the percentages are percentages by weight).
In the case of so-called "low-carbon" sheet, the carbon content must be less than 0.1%, preferably less than 0.03%, with even more preferably a sum of the carbon and nitrogen contents which is less than 0.03%, a manganese content of between 0.03 and 0.3%, a silicon content of between 0.05 and 0.3% and a phosphorus content of between 0.01 and 0.1%. When it is desired to have sheet having a particularly high strength, the carbon content must be less than 0.03% and the manganese content must be between 0.3 and 2%. Additions of boron (up to 0.008%) and of titanium (from 0.005 to 0.06%) into the low-carbon sheet are also possible.
In the case of so-called "ultralow-carbon" sheet, the carbon content must be less than 0.007% and preferably the nitrogen content must also be very low, not exceeding a few tens of ppm. The contents of the other elements are the same as in the case of the low-carbon sheet with, optionally, microadditions of titanium (from 0.005 to 0.06%) and/or of niobium (0.001 to 0.2%).
A process that can replace the above one consists in casting the steel at the exit of the continuous casting mold into thin slab (thickness of 40 to 100 mm, for example) and then in hot rolling it in mill stands in line with the casting plant, this rolling possibly including various steps during which the steel is in the ferritic or austenitic state (see WO 97/46332 incorporated herein by reference). In this process, at least one slab reheat, prior to the first hot rolling, is necessary, as are subsequent cooling and reheating steps allowing the desired metallurgical transformations of the product to be carried out. It is thus possible to produce various types of product, especially sheet with a high formability for the motor-vehicle industry. The present invention is distinct from that described in WO 97/46332.
In the prior processes, using conventional hot rolling mills to obtain the final strip thickness before it is cold rolled, the speed at which the strip leaving the hot rolling plant runs is about 600 to 950 m/min., depending in particular on the thickness of the product. These speeds are relatively high compared with, in particular, the usual speeds at which the products run through the plants to carry out the "cold" treatments of the strip obtained in the rest of the manufacturing process, for example in compact annealing, dip-coating or electroplating lines. This causes differences in productivity between these various plants, differences which require the products to be stored in their intermediate states in the form of coils, while waiting for the "cold" treatments. This results in non-optimal product-flow management, even in the most favorable case in which all the plants--for casting, rolling and "cold treatments"--are grouped together on the same industrial site.